1. Field of the Invention
The present invention relates to a power converter circuit of DC to AC conversion, especially to a conversion circuit, which does not use transformer and transfer the electrical power of distributed energy to electric utility.
2. Description of Prior Art
The request for electrical power is increased as the industry keeps developing. However, the shortage of natural resource and environmental concern for new power plant will make the electrical power more insufficient. Therefore, the renewable energy generation becomes important issue in industrialized or advanced country.
FIG. 1 shows a prior art single-stage three-phase three-wire power conversion circuit. As shown in this figure, capacitors C1 is parallel to DC power source for filtering noise and stabilizing voltage. In this conversion circuit, if the DC power source has common ground with the ground of public network. When the switch Q3 is turned on and the switch Q4 is turned off, the current flows along the path indicated by phantom line. The DC current will flow into the grid, this phenomenon is prohibited in many interconnection rules. Therefore, an output transformer is connected between the power conversion circuit and grid for isolation. FIG. 2 shows power conversion circuit with a transformer arranged on the output side of the power converter therefor providing isolation to AC ground. However, the transformer will increase the size, the cost of the power conversion circuit and reduce conversion efficiency.
FIG. 3 shows a prior art two stage three phase three wire power conversion circuit. In this circuit, both DC side and AC side are grounded, the DC current might flow into the grid through the common grounded points. This phenomenon is prohibited in many interconnection rules. Therefore, an transformer is connected between the power conversion circuit and the grid, as shown in FIG. 4.
FIG. 5 and FIG. 7 which are single stage and two stage power conversion circuits for single phase system, have both-side grounded with potential DC current flowing problem as mentioned above for FIG. 1 and FIG. 3. Therefore, an transformer is connected between the power conversion circuit and the grid, as shown in FIG. 6 and FIG. 8. However, the transformer will increase the size, weight, cost of the conversion circuit and reduce the conversion efficiency.